This invention generally relates to furnace constructions utilized for the melting of ingots and other metal charges. The invention is particularly concerned with melting crucibles utilized in such constructions and with a method for the production of such crucibles.
The costs incurred in the melting of metal charges are greatly influenced by the efficiency of the systems employed particularly in view of the cost of energy. Another major factor in determining such costs is the life of the constructions utilized. Thus, the fact that elevated temperatures are involved tends to accelerate deterioration of furnace constructions, and the problem becomes most acute in the melting of high-melting point materials.
Ceramic melting crucibles are widely employed in the melting of high-melting point alloys including iron, cobalt and nickel base alloys. Such crucibles can withstand the temperatures involved, however, the crucibles are subject to deterioration and periodic replacement is necessary.
In the furnace constructions employing melting crucibles, the crucibles are usually surrounded by packing and liners which separate the crucibles from the heating elements of the construction. Specifically, prior art systems may provide induction heating coils which are embedded within a ceramic liner. In the utilization of this arrangement, a granular ceramic packing is located within the liner with the crucible being received within the packing. The packing and the ceramic mud or the like forming a liner for the construction thus serve as barriers between the crucible and the heating elements. In the event of cracking of the crucible, the barrier acts to prevent the passage of molten material into contact with the heating elements.
The described prior art arrangement tends to be inefficient since a new packing operation must be undertaken each time a crucible is changed. This is a time-consuming operation which contributes to the expense of the melting process. Furthermore, the presence of the granular packing material can also lead to contamination problems which are highly critical when the material being melted is to be utilized in applications requiring parts of the highest integrity.
Prior art crucible designs are also inefficient when the designs utilized for the casting of superalloys are considered. Such crucibles tend to be inefficient for melting purposes, particularly in view of the heat transfer relationship between an ingot charge and the crucible.
FIG. 1 of the drawings illustrates a prior art arrangement of the type described. This construction includes a melting crucible 10 supporting a solid ingot 12. A particulate insulating material 14 is tightly packed around the crucible 10, and an annular cap 16 serves to hold the packing material in place. The packing material is pressed between the interior wall of liner 18 and the wall of the crucible 10.
The liner 18 has induction heating means 20 embedded therein. In accordance with this invention, the ingot 12 is adapted to be located within the crucible for exposure to the heating means. The induction coils are adapted to melt the ingot, and after the melting has been concluded, the entire assembly may be tipped about the trunnions 22 for pouring of the molten metal into casting molds or the like. The cap 16 is designed to prevent the loss of particulate material during such an operation. Since the crucible 10 will gradually deteriorate, it must be periodically replaced, and this necessitates removal of cap 16 and repacking of the packing material after a new crucible is put into position.